Focusing screen for camera finders



K. SCHIELE wocusmc scam FOR cum mums Oct. 10, 1961 5 Sheets-Sheet 1Filed July 8, 1958 FIG! FIG.2

Oct. 10, 1961 K. SCHIELE 3,003,387

FOCUSING SCREEN FOR CAMERA FINDERS Filed July 8, 1958 a? 3 FIG.4

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5 Sheets-Sheet 2 Oct. 10, 1961 K. SCHIELE FOCUSING SCREEN, FOR camFmosns I Filed July 8, 1958 5 Sheets-Sheet 3 FIG] FIG.9

smqle PY Oct. 10,1961 K. SCHIELQE I 3,003,387

FOCUSING SCREEN FOR CAMERA FINDERS Filed July 8, 1958 5 Sheets-Sheet 4Ot. 10, 1961 K. SCHIELE 3,003,387

FOCUSING scam FOR CAMERA FINDERS Filed July 8; 1958 s Sheets-Sheet 5FIG.

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ref/ex finder United States Patent ice 3,003,387 FOCUSING SCREEN FORCAMERA ma Karl Schiele, Braunschweig, Germany, assigns:- to Frllke &Heldecke, Fnln'ik Phoioahphimher Prazisions-Ap par-ate, Bnunschweig,Germany, a German firm C Filcldig uly 8, 1958, No. 74:11:94 195 lalmsprio plicafion y itlalms. ass-1 This invention relates to a focusingscreen for camera finders, especially for reflex cameras. I I

An object of the invention is the provision oia generally improved andmore satisfactory focusing screen.

Another object is the provision of a focusing screen structure forincreasing the apparent brightness of the image as seen on the focusingscreen by the observer, in a simple and inexpensive manner.

Still another object is theprovision of a focusing screen so designed.and constructed as to enable sharp and accurate focusing of the image,and at the same time serving to deflect thcrays of light from theobjective toward the loc s f thc cyeloflthe observer, to increase theapparent brightness'of the image,

e The tollowingdescription prcsuppo that the reader is familiar withthegeneral requirements and general optical principles invoked. inscreens for tocusin viewe finders of reflex cameras or? the like, as Oulined for example in 11.5. Patent 1', 5,72,236, dated February 9, 1926,to d c l ndi lU.-$- Pat 2,589,014. d ed M ch 11, l 95 2 to McLeod,.andin German Patent 753,376, published February 9,1953, to Zeiss Ikon A.G.The

principles andrequirernents setforth in those patents need not berepeated here. Q t

The above-mentionedMcLeod patent emphasizes (colmn lines -13); thatinolder to a hieve t d sircd results, it is necessary to use twoseparateoptical elements, one being a Fresnel lens surface. and the other beinga ni l a e ;-Acc n i s t he pr en in vention it has been discovered,however, that if a conical 0r. yr m su ce is f m d n a pa t cu a w n/Pntented Oct. 10, 1961 FIG. 9 is a diagram illustrating a preferredarrangement of'orientation of pyramids according tothe presentinvention;

FIG. 10 is a diagrammatic axial section similar to FIG. 4, illustratinga diflorentform of pyramidal surface from that shown in FIG. 4,andomitting the forming tool present in' FIG. 4;

FIG. 11 is a diagrammatic axial section illustrating the invention asapplied to a reflex finder;

FIG. 12 is an end view of a forming tool for forming pyramids accordingto the present invention;

FIG. 13 is a sideelevation of the same; and

FIG. 14 is another side elevation of the same,,viewed horn a directionat right angles to th edirection' of viewing in FIG. 13. p

The same reference numerals throughout the several views indicate thesame parts.

Referring now to the drawings, an objective is shown at 8, whichobjective forms an image of the objector: the screen. The objective ismovable toward and away from the screen for focusing purposes, byconventional I mechanisms well known in the art. For purposes of thecones or. pyramids haying apex angies and .axes in a particular a s t e,Ma time t e con cal or pyramidal pattern itself can be made, to performthe functions which. it was previously thought necessary to have e s s si ens I It a dingly, a further object of the presentinvenfionito pros ou n s e PB? Surface which may .b plans e dslmw h a nly. one su ac owhich is Ple e nsn-sm sfluthi te sedat on ain a nml m s at sonn e@Pnsshav na i urs a y metry at increasing the optical axis, or amultitude ofpyramid skeach of which has its. i (from apex omid p l t m'n tilted l a d: mus c *9 e e e Pl of th i u s s n. fi w i s in. an. aum calm easie -J nus ewes m ys atta md in 'the manner illustrativeembodiment of 1.116 nve ion in the Mil ias d s r p n d i the mp n iasdawi ss tan ns a p r er of. i h:

P1655 5 2, udfia e p al d agramsiillustratingthe V f dament l principledthe invention;

3 FIG; 4.,is a diagr mmatic axial section taken through an objective andpart of an focusing screen (showing theJatter on a enlarged scale),illustrating onetorm'otpyramidal surface anil alsoshowins presentinvcntionpit is wholly immaterial whether the objectivetprojects theimage directly onto. the focusing screen, in the manner shown forexample in FIG. 1 o! the drawings of said McLeod patent and FIGS. 1-4and 10 of thefpresent application, or whether the rays of light passingfrom the objective to the focusing screen are, reflected bye. mirror, asshown for example in FIG. 2 or the drawings of said McLeod patent, andFIG. ll of the of the presentapplication' and as customary in reflexcameras. Moreover, it'is wholly immaterial for purposes of the presentinvention ,whetherzthe objective 8 serves both as a photographicobjective and as a iocusing objective, as usual in mailed single lensreflex cameras,

or whether it constitutes merely afinderobjective and is mediaconjunction with a separate photographic objcctive, as a is customary insocalled twin 'lens reflex cameras, such as the Rolleillex" andRolleicord" cameras manufactured by the firm of Franke & Heidecke, ofBraunschweig, Germany, and widely known and'nsed in the United Statesand elsewhere. ,Whatever the arrangement, whether it be non-reflex,single lens reflex, or twin lens reflex, the focusing screen of thepresent invention in any event operates in the same way.

According to the present invention, one of the focusing screen orfocusingpgrid plate maybe smooth, while the other face is provided witha multiplicity of very small conical or pyramidal elements, this lastmen.- tioned face being the one on which the image is focused. In thediagrams constituting FIGS. 1-3, opposite faces of the cones orvpyramids- ,are indicated diagrammatically at 2 and 3,1and they meet atan apex 7. With correct thc image point is exactly in the transverseplane of the apex 7. The component image is then not split up. if theimage point moves out of this correctly focusedposition (i.e.. out ofthe position in the trans: we! Plane of the apex 7) then the conical orpyramidal surfaces 2', 3 cause the splitting up of the light forming theimage, according to known optical principles, so that the imageon thescreen appears blurred. I I

the light energy distribution, the surfaces 2, 3 cause the light to beSPl t up into a number of coni- 'cal beamswhich have the same apexangles as theconical beam between the objective 8 and the surfaces 2, 3.If the surfaces 2, 3 are conical, they cause the light to be I split upintoan infinitely large number of conical beams 4, 5 'whichtsurround theaxis 6. By this means, the light is limited to this total, conical beamformed by theinfinitely large number of conical beams; If the surfaces2', 3 are pyramidal faces instead of conical, the

" light is not-'split'up'intoan infinitely ge number-ct 3 conical beams4, 5, but only into a number of conical beams corresponding to thenumber of pyramidal surfaces.

FIG. 1 illustrates thebeam splitting action when the apex 7 of theconical or pyramidal element is located on the optical axis 6 of theobjective 8. FIG. 2 illustrates the situation when the apex 7 is offsetlaterally to one side of the optical axis 6 of the objective 8, and itwill be noted here that the opposite faces 2 and 3' of the conical orpyramidal element, which meet at the apex 7, are slopednon-symmetrically or asymmetrically with respect to the axis 6aextending from the center of the objective 8 to the apex 7 of theparticular element in question. The slope or tilt is such (that is, ttheangles d and d are so chosen, with respect to the amount of lateraloffset of the apex 7 from the objective axis 6) that the splitting up ofthe light beam forms a group of conical beams 4' and 5 arranged about anaxis 6 which is parallel to the optical axis 6 of the objective 8.

In FIG. 3, the cone is given an even greater inclination or tilt, sothat'all cones of rays are deflected with the angle S in a directiontoward the eye of the observer, assuming that the eye of the observer islocated at a suitable point on the axis 6" corresponding to the opticalaxis 6 of the objective 8.

FIG. 4 illustrates somewhat diagrammatically a more complete structureembodying the principles mentioned in 4 screen will converge toward acommon point 18 located on the optical axis 6, preferably about half waybetween the screen and the eye of the observer. This convergence of theaxes of the various cones does not have to be exact, but convergenceapproximately toward a common point on the optical axis is sufiicient togive superior results as compared with the prior art.

FIG. 6 shows the deflecting surfaces or individual optical elements inthe form of a pyramid having a base 20 in the form of an equilateraltriangle. 7

In FIG. 7, the optical element is shown as a foursided pyramid having abase 21 which is square or rectangular.

The pyramid shown in FIG. 8 has a base 22 in the form of an isoscelestriangle.

By forming the light-deflecting surfaces 2, 3 as cones on an ellipticalbase surface, or as pyramids with a rectangular base surface orisosceles triangle base surface, it

connection with FIGS. 1-3. In FIG. 4, the objective 8 projects thefocused image on the optical axis 6 to the focusing screen or grid plate9. Due to the presence of the cones or pyramids, all the light cone axesare deflected toward the eye 14 of the observer, this being accomplishedby the fact that the axes of the individual cones or pyramids are tiltedor inclined to a progressively greater extent at progressively greaterdistances laterally from the optical axis 6.

A tool 29 for forming the conical or pyramidal elements in the gridplate or focusing screen is also shown in FIG. 4, in various workingpositions, further details of such tool 29 being described below inconjunction with FIGS. 12l4. In position A when forming a conical orpyramidal element at or approximately at the center of the focusingscreen (i.e., on the optical axis) the tool 29 is tilted in such mannerthat the axis of the pyramidal or conical element will be substantiallysymmetrical with respect to the optical axis 6. When forming pyramidalor conical elements somewhat oflset laterally from the optical axis, inthe position A the tool 29 is tilted differently so as to tilt the facesor sides of the cone or pyramid to deflect the light beam to a directionconverging toward the optical axis 6, as indicated for example by theline 6b, so as to pass toward the eye 14 of the observer. When formingother pyramidal or conical elements further offset laterally from theoptical axis and-closer to the marginal edges of the focusing screen,the tool is tilted to a different angle as indicated at A soas to causea greater tilt or inclination of the sides of the conical or pyramidalelements relative to the optical axis, deflecting the light beam to agreater extent so that the emergent light passes in the direction do,again toward the eye 14 of the observer. In each position of the formingtool, regardless of the tilt or inclination of the tool, the tool ispreferably pressed against the surface of the screen in the directionshown by the arrows; that is, in a direction perpendicular to thegeneral plane of the screen.

Reference is now made to FIGS. 58, showing dia: grammatically some ofthe possible shapes of the individual conical or pyramidal elements.FIG. 5 is intended to illustrate a conical element having a conicalsurface 15, the base 16 of which may be either circular or elliptical.The axis of the cone is illustrated at 17, and it may be either at rightangles (perpendicular) to the plane of the base, or may be inclinedthereto. When conical elements are used according to the presentinvention, the axes 17 of the various cones, in various positions on theis possible to achieve a preferred splitting of the light in thedirection of the longitudinal axes of the base surfaces. This isdesirable especially for the marginal elements of the foscusing screen,owing to the different width of the entrance pupil aperture when viewedat an angle to the optical axis of the objective. .This is shown in FIG.9, indicating diagrammatically the face of the screen having pyramids 24of the kind shown in FIG. 8; that is, pyramids the bases of which areisosceles triangles. These pyramids in FIG. 9 are arranged with theirsmall faces all directed toward the center of the screen 9.

It will be observed that the preferred form of the lightdeflectingsurfaces 2, 3 on the screen is that of a pyramid having a triangularbase, as this gives better light control than cones. Also, pyramids withtriangular bases are superior to those formed on bases having more thanthree sides, because pyramids with more than three faces are likely tobe less effective, due (to the fact that the edges or corners betweenany two adjacent faces of the pyramid will unavoidably be slightlyrounded in the manufacturing process. If the pyramids have more thanthree faces, the rounded edges will constitute a larger proportion ofthe total surfaces present, than if each pyramid has only three faces.This would decrease the efliciency of the screen.

For the same reason, it is desirable that the number of pyramids perunit of area of the screen should not be too large, since the roundededges may become too large a proportion of the total area, if thepyramids are too small. On the other hand it is desirable that thepyramids should not be too large, as they would then become visible,especially when a magnifying viewfinding lens is used in conjunctionwith the focusing screen. It is therefore preferred that the distanceapart of the apices of adjacent pyramids in all directions should bebetween 20p. and 40 or in other words from 0.02 mm. to 0.04

mm. I

The cones or pyramids may be formed either as cavities in the screen(i.e., with the apex of each cone or pyramid pressed into the screen,forming a concave pyramid or cone) as shown in FIG. 4, or asprotuberances (i.e., with the apex of each cone or pyramid pointedoutwardly from the body of the screen, to form what may be called aconvex cone or pyramid) as shown in FIG. 10. Also, the cones or pyramidsmay be formed on either face of the screen 9, that is, either facingtoward the objective and away from the eye of the observer, or facingaway from the objective and toward the eye of the observer. Thepreferred arrangement is the one shown in FIG. 4,

employing concave pyramids.

The cavities are preferably made by reciprocating a steel tool againstthe material of the screen, and by changing the angle of the tool inproportion to the distance of each individual cavity from the center ofthe screen, as already mentioned in connection with FIG. 4. The cavitiesmay be'made in one or more spirals, in straight lines, in wavy lines, ormay be arranged in any other desired smear different on dilerentpyramids at different locations on V the screen.

As already mentioned, it is immaterial whether the focusing screen ofthe present invention'be used in a reflex viewfinder, or in a viewfinderof the non-reflex type.

a reflex i fi e will. be emp y d, as c 1 in most modern cameras. Such aviewfinder is in. FIG. 11, where the objective 8 throws the in'ragemto areflector onmlrror 27-, from which it is reonto the screen? constructedin accordance the present invention.-

Some cameras have a magnifying lens which may be interposed at willbetween the focusing screen 9 and the eye 14 of the observer. Such alens is shown at 28 in FIG. 11. When this lens is used, the observerseye is spaced at a different distance from the screen, than when viewingit without the lens. Accordingly, the inclination of the axes of thecones or pyramids is arranged, in such a case, to converge the lightfrom the screen toward a point which is a compromise between the twopositions of the observers eye, when viewing the screen with and withoutthe lens.

It would be diflicult to make focusing screens accord- ,ing to thepresent invention from glass. Therefore it is preferred to make them ofa sheet of transparent synthetic plastic material sufliciently soft toable to receive the indentations of the forming tool, and sufficientlyhard to retain its shape well after the indentations have been made. Asheet of polystyrol plastic, known also as polystyrene, has been foundto be quite satisfactory.

The cones or pyramids are preferably so formed that the apices of all ofthe pyramids or cones normally lie in the same plane, and the screen asa whole is normally used in a fiat condition.

As already explained above, it is not necessary to use a field lens or aFresnel lens in conjunction with the screen, because the screen of thepresent invention performs the function of converging the light towardthe cye of the observer, as well as the function of indicating theout-of-focus condition. However, a field lens or Fresnel lens can beadded if required for special purposes, as for example when an unusuallypowerful magnifying lens is needed.

A typical form of tool 29 for forming the pyramidal elements is shown inFIGS. 12, 13, and 14. This tool may be made of steel, provided withpyramidal surfaces 30, 31, and 32, meeting at an apex 33, the face 32being smaller than the faces and 31, so that the pyramid has a base inthe form of an isosceles triangle, as shown in FIG. 8. The angle B (FIG.14) may be, for example, from 11 to 14 degrees; the angle C (FIG. 13)may be from 28 to 34 degrees; the angle D may be '50 degrees; and theangle E may be 55 degrees.

In speaking of the bases of the conical or pyramidal elements, it is tobe understood'that reference is made to the theoretical bases of thecones or pyramids considered as geometrical figures. In most forms ofthe present invention, these bases do not actually exist as separateidentifiable faces or areas, but exist theoretically or abstractly asparts of the geometrical figures.

For lack of a suitable generic word which includes both cones andpyramids, it is intended that the expression "cone-like andpyramid-like" as used in this application shall be interpreted toinclude cones and pyraraids.

to no. u W'lhkhill nevirear ism srrs meters ands screen-9.4160 6 (FIGS.lsnd'3) of'the near the center ofthe in. M I" aeao-itaesw and etalaterally tromtheoptical are se om a=w and x i= i i 'i ="V These angleswould of have to vary somewhatin accordance with the index of refractionof the plastic material from which the screen is made and given incorrespondence for example with an index of refraction of 1.5.

What is claimed is:

1. A focusing screen comprising a sheet of transparent material havingone face thereof provided with a large number of relatively smallpyramid-like formations the geometrical axes of which formations areinclined progressively from a central area of the screen outwardlytoward the lateral edges thereof, to converge light beams received fromone side of the screen toward a common positionon the other side of thescreen, the apices of all of said pyramid-like formations lyingsubstantially in a common plane and being spaced from the respectivenext adjacent apices by distances from 0.02 to 0.04 millimeter, theapexregion of each of said formations being substantially identical withthe apex region of the other pyramidlike formations in shape but beingoriented differently in such fashion that a corresponding side of eachof said from the body of said transparent material.

6. I A focusing screen comprising a sheet of transparent material havingone face thereof smooth and plane and the otherface thereof providedwith a large number of relatively small indentations of pyramidal form,each of said indentations having an apex spaced from the apices ofrespective surrounding indentations by a distance from 0.02 to 0.04millimeter, the apices of all of said indentations lying in a commonplane parallel to said smooth plane face of the screen, the geometricalaxes of the respective pyramidal indentations from the center of saidscreen outwardly toward the marginal edges thereof being progressivelyinclined to an increasingextent as the dis tance from the center of saidscreen increases, in such manner as to converge light beams receivedfrom one side of the screen toward a common position on the other sideof the screen, said indentations being in the form f of faces ofpyramids having triangular geometricalbases, so that each indentationhas three light deflecting faces,

one of said three faces being smaller than the other two and being facedtoward the center of said screen.

7. A focusing screen for use in focusing a movable 7 lens of aphotographic camera by visual observation of an image projected ontosaid screen by such lens, said screen comprising a sheet of transparentmaterial having one face thereof substantially smooth and plane andhaving the opposite face thereof formed with a great multiplicity ofvery minute pyramid-like formations each having an apex, the apices ofsubstantially all of said formations lying substantially in a commonplane constituting the focal plane of the image to be focused, the sidesof said formationsconstituting refracting surfaces for refracting lightfrom an image which is not focused a'ccu-. rately on the common plane ofsaid apices in such manner as to cause such inaccurately focused imageto appear blurred in contrast to the appearance of an image sharplyfocused on saidcommo nplanq each of said'formations having a geometricalaxis passing through'its apex, the axes of the formations progressivelyfarther from the center of said screen being tilted at progressivelygreater angles to an optical axis perpendicular to said common plane atthe center thereof, so that light rays from portions of said screenprogressively farther from the center thereof will be refractedprogressively more'towardsaid optical axis, to increase the apparentbrilliance of mar.-

ginal portions of said focusing screen and to tend to overcome the usualdimness of the marginal portions-of a focusing screen as compared withthe central; portion thereof, without requiring a supplementary fieldlens.

8. A focusing screen as defined in claim 7, in which the pyramid-likeformations are of pyramidal shape with one face smaller than the otherfaces and so oriented that the smaller face of each pyramid is facedtoward said optical axis passing through the center of the screen.

References Cited in the file of this patent UNITED STATES PATENTS McLeodMar. 11, 1952

